1. Field of the Invention
The invention relates to a microwave transformer device and, in particular, to a balanced-to-unbalanced transformer device. This type of device is commonly referred to as a balun.
2. Description of Related Art
A balun is often used when it is desired to couple a balanced system or device to an unbalanced system or device. A typical example is the coupling of a two-line (balanced) circuit, such as a cellular telephone circuit, to a single-line (unbalanced) circuit, such as an antenna circuit. Another example is as a signal splitter/phase shifter for use with a balanced mixer.
In some uses, such as in portable cellular telephones, it is important that a balun meet three criteria. It must be compact, have a minimum insertion loss, and have a narrow passband to minimize power wastage. Although prior art baluns are known which accomplish one or two of these objectives, none are known which satisfactorily accomplish all three.